Cable operators have widely deployed high-speed data services on cable television systems. These data services allow subscriber-side devices, such as personal computers, to communicate over an ordinary cable TV network Hybrid Fiber Coax (HFC) cable. Cable Television Laboratories, Inc. (CableLabs®) publishes detailed technical specifications for such systems, including DOCSIS—Data Over Cable Service Interface Specification. In typical cable systems, a Modular Cable Modem Termination System Core (M-CMTS Core) connects the cable network to a data network, such as the Internet. A downstream Universal Edge QAM (UEQAM) located in the cable network receives data transferred from the M-CMTS Core over a packet switched portion of the network (which can be characterized as having variable transmission delays), performs modulation and other processing, and then transfers the ITU-T J.83 modulated data over a Hybrid Fiber Coaxial (HFC) portion of the cable network, which is specified as having a constant transmission delay.
DOCSIS specifies that the cable modems obtain upstream bandwidth according to a request/grant scheme. A cable modem sends a bandwidth allocation request when it receives a packet from a subscriber device and the packet needs to be sent upstream into the cable network. The M-CMTS Core grants these requests using bandwidth allocation map (“MAP”) messages. The MAP contains information that indicates when a cable modem can transmit and for how long. The modem then waits for its scheduled time before it can transmit. This cycle is referred to as the request-grant cycle. The maximum number of transmit bursts per second that an individual cable modem can send is inversely proportional to the request-grant cycle duration. To minimize that cycle time, the CMTS must ensure that the MAP messages reach the Cable Modems in a timely manner, despite numerous competing factors that can impact the generation and delivery of MAP messages, especially in the M-CMTS systems where the M-CMTS Core and UEQAM are separated by a variable-delay network. The present disclosure in one aspect enables dynamic delay adjustment for MAP messages in an M-CMTS environment.
Recently, some systems have employed a Universal Edge QAM or (“UEQAM”). A UEQAM processes both video and DOCSIS data. In the following description and the appended claims, we use the term EQAM broadly to include both UEQAM and EQAM.